1. Field of the Invention
The present invention relates to a tensioner used for imparting a proper tension to a timing belt of an automotive engine or a belt for driving an auxiliary machine such as an alternator or compressor.
2. Description of the Related Art
Conventionally, the automatic tensioner is used in a transmission apparatus for transmitting a drive force of an engine to a plurality of devices by a single endless belt. The tensioner imparts a suitable tension to the belt and, at the same time, damps vibration of the belt generated due to fluctuation of the engine rotational speed or load. Thus, the drive force of the engine is reliably transmitted to the devices.
The tensioner is fixed to an engine block or in the vicinity thereof and is provided with, for example, a fixing member on which a cup is formed, an arm rotatably supported by a shaft so that it can freely rotate with respect to the cup, and a pulley attached to the front end of this arm. The arm is rotated, in a direction which presses the pulley against the belt, by a torsion coil spring provided substantially concentrically with respect to the center of rotation of the arm, by which a suitable tension is imparted to the belt. Further, as a damping mechanism for damping the vibration of the belt, a frictional member is provided between the arm and the cup. Due to this frictional member, a rotational resistance, i.e. damping force, is produced at the time of rotation of the arm, thus braking the rotation of the arm, and the vibration of the belt is damped.
In order for the pulley to constantly abut the belt and effectively damp the vibration of the belt, the arm is required to rotate relatively quickly in a direction in which the belt becomes tensioned and relatively slowly in a direction in which the belt becomes loosened. However, the above friction type damping mechanism is constituted so as to impart the same damping force in both directions, so the arm cannot rotate quickly in the direction tensioning the belt. As a damping mechanism for quickly tensioning the belt, for example, an oil pressure type damping mechanism in which an oil pressure cylinder is provided in place of the frictional member and a viscous resistance of oil is produced in only the direction in which the belt loosens is known.
However, the oil pressure type damping mechanism has a large number of parts and a complex structure in comparison with the friction type damping mechanism, therefore the cost becomes high. Further, the viscosity of the oil depends upon the environmental temperature, so the damping force is reduced in a high temperature region in comparison with that at room temperature, and the viscous resistance becomes high in a low temperature region and therefore the damping force is increased. Further, the direction of attachment is limited so as to prevent oil leakage, therefore the degree of freedom of design of the tensioner as a whole is lost and reduction of size becomes difficult.